1. Which of my pieces are 'loose'? As Nunn says, 'LPDO'. Loose Pieces Drop Off.
2. What are my opponent's threats?
3. What's going on where the action isn't? Often players focus on one part of the board where they think the most important changes are taking place, and overlook the implications for the rest of the board.
What questions should I ask myself so I most likely make a bit less obvious mistakes? Those are called yawns if I remember correctly